A simple and rapid on-line analytical system was configured for the evaluation of the photostability of various food coloring agents constantly exposed to light. The screening test for photostability of food coloring agents was carried out in the system consisting of 60 cm of Teflon tubing for ultraviolet (UV) irradiation with a flow rate of mobile phase (water/acetonitrile = 1:1, v/v) at 1 mL/min. When the products derived from photochemical reaction of phloxine (R-104), which is recognized to exhibit chemiluminescence induced by UV irradiation, were separated, several peaks were confirmed on the chromatogram in this system. Furthermore, the peak of chemilumigenics observed on the chromatogram of photon-counting chemiluminescence detection corresponds to the new peaks on the UV chromatogram. Also, the presence of phenoxy radical (g = 2.0061) formed by UV irradiation of R-104 was confirmed with an electron spin resonance spectrometer, indicating a reasonable relationship (r = 0.9041) between the formation...